Many different types of sensors are manufactured as micro-machined electromechanical system (MEMS) devices in different crystalline materials, including by example and without limitation, crystalline silicon and other crystalline materials that react similarly to conventional MEMS manufacturing techniques. One example of conventional MEMS manufacturing techniques is a silicon on insulator (SOI) process commonly used in the manufacture of sensors, particularly force-versus-displacement or “force/displacement” sensors for measurement of acceleration. Other MEMS devices are manufactured using an epitaxial wafer process and even sensors formed in undoped silicon wafers having no oxide growth at all.
For purposes of laying a background for the present invention, one typical example of a prior art (MEMS) device is provided to illustrate the common cantilever-style strain isolation device of the prior art.
Accelerometers generally measure acceleration forces applied to a body by being mounted directly onto a surface of the accelerated body. One common type of accelerometer produced using conventional MEMS manufacturing techniques employs one or more force-versus-displacement or “force/displacement” sensors for measurement of acceleration. Accelerometers employing two force/displacement sensors instead of the necessary minimum one sensor gain considerable advantage. If the two sensors operate in a push-pull mode, then many error sources such as thermally driven effects or drift may be rejected as common mode, while the difference signal represents the desired acceleration measurement. Occasionally, designs using two force/displacement sensors include two completely separate proof masses, which results in essentially two accelerometers, each having its own sensor, but operating in opposite directions. For numerous reasons, however, a two proof mass solution is not preferred. Rather, it is generally advantageous to have only one proof mass in an accelerometer.
Many different types of force/displacement accelerometers are manufactured as MEMS devices using conventional techniques. One typical example of a prior art MEMS device is a micromachined two-sensor/single proof mass accelerometer. By example and without limitation a MEMS accelerometer device, commonly referred to as a Rectangle design, is provided to illustrate the current state of the art.
FIGS. 1A, 1B, 1C and 1D therefore illustrate by example and without limitation an exemplary MEMS accelerometer 10 as a miniature structure fabricated from a substrate 12 of semiconductor material by conventional micromachining techniques. The substrate 12 is formed of a monocrystalline silicon material in a substantially planar structure, i.e., having substantially planar and parallel opposing offset upper and lower surfaces. The silicon substrate 12 often includes an upper silicon or active layer 14 that is electrically isolated from an underlying substrate 16 by an insulating layer 18, or an insulating layer is applied to active layer 14, as shown and described in U.S. Pat. No. 5,948,981, Vibrating Beam Accelerometer, issued Sep. 7, 1999, and assigned to the Assignee of the present application, the entirety of which is incorporated herein by reference. The insulating layer 18 is may be a thin layer, e.g., about 0.1 to 10.0 micrometers, of an oxide, such as silicon oxide. The silicon substrate 12 is usually formed by oxidizing active layer 14 and underlying substrate 16, and adhering the two layers together. A portion of active layer 14 may be removed to bring the layer 14 to the desired thickness. The silicon oxide layer 18 retains its insulating properties over a wide temperature range to ensure effective mechanical resonator performance at high operating temperatures on the order of 100 degrees Celsius. In addition, the insulating layer 18 inhibits undesirable etching of the active layer 14 during manufacturing.
The micromachined accelerometer 10 includes an acceleration sensing mechanism 20 having one or more flexures 22 pliantly suspending a proof mass 24 from an inner sensor frame or plate 26 for movement of the proof mass 24 along an input axis I normal to the proof mass 24. The flexures 22 are preferably etched near or at the center of the underlying substrate 16, i.e., substantially centered between the opposing upper and lower surfaces of the underlying substrate 16. Optionally, the flexures 22 are formed by anistropically etching in a suitable etchant, such as potassium hydroxide (KOH). The flexures 22 define a hinge axis H about which the proof mass 24 moves in response to an applied force, such as the acceleration of the accelerated body, for example, a vehicle, aircraft or other moving body having the accelerometer 10 mounted thereon. The sensing mechanism 20 includes a pair of force/displacement sensors 28 coupled between the proof mass 24 and the sensor frame 26 for measuring forces applied to the proof mass 24. The force/displacement sensors 28 are, for example, mechanical resonators formed from the active silicon layer 14 as double-ended tuning fork (DETF) force sensors.
In response to an applied force, the proof mass 24 rotates about the hinge axis H, causing axial forces, either compressive or tensile, to be applied to the mechanical resonators 28. The axial forces change the frequency of vibration of the mechanical resonators 28, and the magnitude of this change serves as a measure of the applied force or acceleration. In other words, the force/displacement sensors 28 measure the applied acceleration force as a function of the displacement of the proof mass 24.
Top and bottom cover plates 30a, 30b are used as damping surfaces and shock stop restraints. Undesirable external stresses and strains may be induced in the sensitive acceleration sensing mechanism 20 by mechanical coupling of the accelerometer sensor frame 26 to the pair of top and bottom silicon cover plates 30a, 30b, one of which in turn is typically mechanical coupled to the ceramic or metal mounting plate 32. A major problem consistently confronting the designer of high performance accelerometers relates to supporting the sensing mechanism 20 without locally introducing additional error sources due to discontinuities at the interface between the accelerometer sensor frame 26 and the cover plate 30. These discontinuities are typically introduced in the form of bonding agents or fasteners formed of a different material from that of the sensing mechanism 20.
The available bonding agents, such as epoxy or a glass frit, exhibit thermal expansion coefficients substantially different from the silicon substrate of which the sensing mechanism 20 is formed. The bonding agents are usually cured at elevated temperatures, which results in an internal stress condition between the silicon and the bond joints. The bonding agents also exhibit other different physical characteristics that combine to produce localized stress and mechanical hysteresis at the interface. The localized stresses and mechanical hysteresis must be isolated from the sensor mechanism to prevent errors in the sensing function. Any strains occurring in the sensor frame 26 are transmitted not only to the proof mass 24, but through the proof mass 24 to the two DETF resonators 28. Since the only significant compliance in the system is the sensing DETF resonators 28 themselves, almost the entire strain appears as an error output from the DETF resonators 28. Thus, undesirable errors are generated in the DETF resonators 28 from inputs having nothing to do with the acceleration being measured. These errors can be quite large since the compliance through the DETF resonators 28 must be low to detect acceleration with sufficient accuracy to be useful in practical systems.
Strain isolation within the micro-machined accelerometers is thus of paramount importance for good performance, i.e., accuracy. Strain isolation separates the mechanism from stresses mechanically induced during fabrication and assembly, and thereby reduces variations in resonance within the beams of the two vibrating-beam force sensing portions of the accelerometer mechanism. Strain isolation also separates the mechanism from stresses externally induced by shock, vibration and temperature variation within the operating environment.
Many methods are known for isolating the sensitive acceleration sensing mechanism 20 from such undesirable stresses and strains. Typically, the acceleration mechanism is suspended on a frame and the cover bond joints are formed on a peripheral rim connected to the frame with suspension beams. For example, cantilever-style isolation is provided wherein the sensor frame 26 is suspended from a second outer or external frame portion 34 by flexures 36 formed by overlapping slots 38 and 40 through the substrate 12. The sensor frame 26 is thus able to move relative to the outer frame 34, as shown and described in U.S. Pat. No. 5,948,981, which is incorporated herein. Such isolation minimizes the distortion of the sensor frame 26, and thereby decreases the effects of external stresses and strains on the mechanical resonators 28.
FIG. 1B illustrates assembly of a die stack, whereby the top and bottom cover plates 30a, 30b are bonded to the second outer or external frame portion 34 along their peripheral edges to form the completed accelerometer 10, commonly referred to as a “die stack.” Top and bottom cover plates 30a, 30b are used as damping surfaces and shock stop restraints. The accelerometer or die stack 10 in turn is typically adhesively connected to the ceramic or metal mounting plate or a header 32 with appropriate drive electronics attached to form the completed accelerometer.
FIG. 1C is a cross-section view taken through the micromachined accelerometer 10 along the resonators 28. As discussed above and shown in the Figures, the proof mass 24 is free to rotate about the flexures 22 when subjected to acceleration along the input axis I according to the principle of Newton's law: F=ma. This rotation is constrained by the action of two force/displacement sensors 28, shown as DETF resonators, positioned on a surface of the mechanism as shown. These two vibrating beam force sensors 28 provide push-pull variable frequency output signals since, when the proof mass 24 is displaced relative to the plane of the sensing mechanism 20, one DETF resonator 28 is under compression while the other is under tension. The difference between the two frequencies represents the measured acceleration. Common mode frequency shifts, on the other hand, are rejected as errors driven by unwanted sources such as temperature, mechanism stress, or drift.
As illustrated in FIG. 1C, the top and bottom cover plates 30a, 30b are bonded to the second outer or external frame portion 34 along their mutual edges to form the completed accelerometer or die stack 10. The inner sensor frame or plate 26 having the proof mass 24 suspended therein is thereby suspended in turn between the top and bottom cover plates 30a, 30b by the flexures 36. FIGS. 1A-1C thus demonstrate the cantilever-style isolation provided by the prior art.
A known oscillator circuit, shown in FIG. 1D and described in above-incorporated U.S. Pat. No. 5,948,981, drives the mechanical resonators 28 at their resonance frequency. FIG. 1D illustrates a representative oscillation circuit 50 in which vibrating beams of the transducers 28 function as a resonator. A transimpedance amplifier 52 converts a sense current received from vibrating beams to a voltage. This voltage is filtered by a bandpass filter 54, which reduces noise, and the voltage amplitude is controlled by an amplitude limiter 56. The resulting signal is combined with the output or DC bias voltage from a DC source 58 in a summing junction 60. The DC bias voltage generates a force between electrodes and the beams of the force/displacement sensors 28. The signal from amplitude limiter 56 modulates this force causing the beams of the transducers 28 to vibrate laterally at their resonant frequency. This lateral beam motion, in turn, generates the sense current. An output buffer 62 isolates the oscillator from external circuitry connected to an output 64 of oscillation circuit 50. The gain in oscillation circuit 50 sustains oscillation of the beams of the force/displacement sensors 28.
Prior art MEMS designs have effectively used the cantilever-style strain isolation, new applications continually reduce the space available for the accelerometer. New constraints are placed upon the space available within the accelerometer for strain isolation. These new space constraints do not permit the cantilever-style strain isolation of the prior art. Accelerometer designers are thus challenged in providing sufficient strain isolation within minimum spacing.
FIGS. 2A, 2B and 2C illustrate one effective strain isolation technique. The strain isolation technique disclosed by U.S. Pat. No. 6,301,966, CLAMSHELL COVER ACCELEROMETER, issued Oct. 16, 2001, to the inventor of the present invention and assigned to the Assignee of the present application, the complete disclosure of which is incorporated herein by reference, provides a direct reduction of driving stress and improved isolation by replacing conventional cover plates with “clamshell” cover plates, whereby that the sensing mechanism is housed within the cover plates. A single cover-to-cover bond on the centerline of the cover plate bonding areas bonds the plates to each other. Bonds for securing the sensor mechanism are optional. If present, the sensor mechanism bonds are localized and isolated from the sensing mechanism. Thus, the clamshell design solves the problem of internal stresses more effectively and less expensively than other prior art isolation structures.
FIGS. 2A-2C illustrate the strain isolation technique of U.S. Pat. No. 6,301,966 that eliminates the cantilever-style strain isolation and the second outer or external frame and the cantilever-style flexures 36 suspending the sensor frame 26 and the sensitive acceleration sensing mechanism 20, as illustrated in FIGS. 1A-1D. FIGS. 2A and 2B illustrate the clamshell accelerometer 75 having a pair of clamshell cover plates 76 and 78 structured to accommodate an accelerometer/sensing mechanism 80. The clamshell cover plates 76 and 78 include deeply etched cavities 82 and 84, respectively, that permit motion of the accelerometer's proof mass 86 and provide space for the entire sensing mechanism 80. The sensing mechanism 80 is thus entirely enclosed inside the cover plates 76 and 78, with the cover plates 76 and 78 bonded directly to each other by a centerline bond 88. Small tabs 90, 92 and 94 on the sensing mechanism 80 are bonded to the cover plates 76 and 78. The bonds at tabs 90, 92 and 94 are very small, localized and positioned for minimum sensor impact.
The single centerline bond 88 between the cover plates 76 and 78 reduces cover bonding to a single joint and half the bond material, which directly reduces the driving stress. The clamshell cover-to-cover centerline bond 88 also eliminates mismatch between top and bottom bond joints that will otherwise warp the sensing mechanism out of plane.
However, while the clamshell cover invention of U.S. Pat. No. 6,301,966 markedly improves strain isolation, the sensing mechanism 80 must still be constrained within the confines of the clamshell cover plates to avoid large bias and scale factor errors as well as alignment shifts. One constraint mechanism is limiting the amount of bonding agent at the selected localized sites between the cover plates and the sensing mechanism. Unfortunately, even small amounts of bonding agent introduces an undesirable contact with foreign material at the sensing mechanism interface.
Alternatively, the bond points 90, 92 and 94 may be replaced by contact pressure holding the sensing mechanism 80 in place between the clamshell covers 76 and 78, as illustrated in FIG. 2C.
However, the clamping force required to constrain the sensing mechanism through friction at the interface requires impractically high tolerances while also placing undue stress on the sensing mechanism. Furthermore, large clamping forces will also impact performance and long term drift as the stress relieves over time and environmental exposure.
Accelerometer and other MEMS device designers thus to be challenged in providing effective strain isolation within minimum spacing.